Mash feed conditioning apparatuses, roller mills and pellet mills, as well as processes for pelleting particulate feed material using such devices individually, are old in the art. The processing of particulate feed material is, nevertheless, quite complex. In order to process feed efficiently, it is important to take many factors into account during the design and operation of a pelleting system. Of critical importance to such design and operation is the configuration of the mash feed conditioning apparatus employed.
Typically, a mash feed conditioning apparatus employs heat, which can be supplied in various forms, to heat condition or "cook" the mash feed prior to pelleting. Often, steam is used to transfer heat and moisture to the mesh feed during conditioning operations. The utilization of steam to transfer heat, however, presents several problems. Of primary concern is the tendency of steam conditioning to impart too much moisture to the feed being conditioned. When the mash feed becomes too moist, it often takes on a tacky or sticky consistency which is nonconducive to the conditioning process. For example, "sticky" feed adheres to and clogs the internal assemblies of the conditioning apparatus, the roller mill and the pellet mill. Thus, when "sticky" feeds are pelletized, most pelleting systems must be stopped frequently for cleaning and unclogging. This results in a lowered throughput due to increased "down time" during normal pelleting operations.
In an attempt to reduce the amount of moisture introduced into the feed, conditioning apparatus operators often reduce the residence time of the mash feed within the conditioning apparatus or reduce the amount of steam introduced. However, this strategy results in a reduction in the heat transferred to this mash feed and, ultimately, in insufficiently conditioned feed. It is well appreciated in the art that to make a better quality pellet, the temperature of the mash feed must be elevated as high as possible without burning or drying the feed prior to pelleting the feed. Furthermore, lack of sufficient heat conditioning reduces the degree of gelatinization of starch within the feed and, accordingly, the nutritional value of the feed, particularly in respect to new born monogastric animals such as baby pigs which generally have insufficient digestive enzyme producing capability to completely digest ungelatinized starch.
Another strategy involves maintaining the interior of a conditioning apparatus at superatmospheric pressures. However, direct contact between the steam and the mash feed, and the steam and the apparatus walls without prior adjustment to the lower pressure in the superatmospheric conditioning apparatus still results in the transfer of a significant amount of condensed moisture to the feed in the apparatus. For instance, Williams (U.S. Pat. No. 4,001,452) discloses a conditioning apparatus wherein the mash feed is conditioned by steam maintained at superatmospheric pressure within the apparatus. However, as noted above, this system disadvantageously exposes the mash feed directly to the steam being introduced therein. Similarly, White (U.S. Pat. No. 4,667,418), which discloses an anaerobic conditioning system utilizing high pressure steam which also exposes mash feed directly to the steam. Thus, mash feed being conditioned in either of the above apparatuses or systems will continue to present the difficulties associated with the overmoist feed discussed above.
It is clear that there has existed a long and unfilled need in the prior art for a mash feed conditioning apparatus which can be used to condition mash feed with an optimal amount of steam heat without introducing too much moisture to the feed to allow for effective pellet processing of the conditional feed. In particular, a need exists for a mash feed conditioner which optimizes the amount of heat transferred to the feed with every respective unit of moisture. Furthermore, a need exists for a method of conditioning mash feed with steam without imparting excess moisture to the feed being conditioned. The present invention solves these and other problems associated with the prior art.